Atrial fibrillation (“AF”) is a common example of an atrial tachyarrhythmia, which is characterized by having a fast atrial rate and a variable ventricular rate. In AF, the regular electrical impulses that are generated by the heart's sinoatrial (“SA”) node are interfered with, or replaced, by other less regular and more rapid reentrant electrical impulses. Thus, AF results in irregular heart beats. Typically, AF is not life threatening, however, the likelihood of stroke is increased in individuals that experience AF.
In order for AF to exist, the surface area of the atria must be relatively large and contiguous. Surgical procedures that subdivide the conducting tissue of the atria can eliminate AF. An example of one such procedure is the Maze procedure, in which the atria are surgically segmented during an open-heart surgical procedure so that reentrant electrical signals can not propagate through the atria. During the Maze procedure, a surgeon creates a series of linear incisions in the surface of the atria, resulting in a mazelike pattern of scars. The surgically created scars electrically subdivide the atria in such a manner that AF can not be supported.
Other procedures for eliminating AF involve the use of an ablation catheter. The ablation catheter is inserted into a patient's heart, typically, via the patient's venous system, e.g., through the patient's jugular vein, without the need for open-heart surgery. During one type of ablation procedure, a medical practitioner moves the ablation catheter until the distal end of the ablation catheter, which includes an ablation electrode, contacts heart tissue that is to be ablated. Next, the medical practitioner applies energy, e.g., radiofrequency (“RF”) energy or microwave energy, to the ablation electrode, which, in turn, creates thermal energy at the distal end, which scars the heart tissue. In other types of ablation catheters, the distal end includes a piezoelectric crystal that is configured to transmit ultrasound energy into heart tissue that contacts the distal end. The transmission of ultrasound energy into the heart tissue creates thermal energy, which scars the tissue.
Typically, the ablation energy heats the heart tissue to a temperature ranging from approximately 65° C. to approximately 100° C. for between 10 to 60 seconds. By moving the ablation catheter's distal end while applying thermal energy to the heart tissue, the medical practitioner creates linear scars in the heart's tissue, which subdivide the surface area of the atria in a manner similar to that of the Maze procedure. In practice, the effectiveness of the scars created using the ablation catheter approximate the effectiveness of the scars created during a Maze procedure.
In another ablation procedure, the ablation catheter is used to destroy only a limited region of the heart's tissue. More specifically, in an effort to control the rate of the heart, the atrioventricular (“AV”) node is ablated and a pacemaker, which controls the rate of the patient's heart rate, is implanted. In other ablation techniques, one or more cells, also referred to as “focal points” are ablated. Typically, these focal points are located in the left atrium near the pulmonary vein, and believed to be the origination point for AF. These focal points, as well as reentrant circuits, in the heart are referred to as “drivers.” The drivers are referred to as “AF drivers” when the drivers initiate AF. These AF drivers can provide a single, stable electrical activation that drives an atrium for a very short cycle length, e.g., a cycle length on the order of approximately 100 milliseconds to approximately 200 milliseconds.
Ablation techniques are most successful when the ablation energy is delivered to the heart's tissue in an accurate manner. Currently, the medical practitioner may map the response of the heart to an applied stimulus in an effort to accurately determine the location for the delivery of the ablation energy. During this mapping procedure, the ablation electrode can be used to electrically stimulate various locations in the heart. The medical practitioner compares the resulting electrocardiogram (“ECG”), which is detected using external electrodes, to intracardiac signals detected using the ablation electrode.
If the ablation electrode is co-located with the AF driver, the ECG and intracardiac signals detected after stimulation will match the ECG and intracardiac signals that result during an AF event. In this manner, the medical practitioner can determine whether the ablation electrode is co-located with the AF driver by carefully comparing the ECGs and the intracardiac signals that result from the mapping procedure as the ablation electrode is moved along the interior surface of the heart. This comparison of the ECGs and intracardiac signals can be tedious and time-consuming for the medical practitioner because the differences in the timing and shape of the ECG and intracardiac signals can be subtle. After the medical practitioner determines that the ablation electrode is co-located with the AF driver, the medical practitioner will prompt the ablation electrode to transmit energy into the heart tissue that contacts the ablation electrode in an effort to destroy the driver.
Another technique for treating AF involves the use of implantable medical devices (“IMDs”), e.g., pacemakers, cardioverters, and/or defibrillators, which are surgically implanted into a patient and configured to sense the occurrence an AF event. Also, the IMDs are configured to deliver pacing therapy and/or shock therapy to the atria via lead electrodes that are coupled to an IMD in an effort to treat the AF event and to restore normal cardiac rhythm. The ability of the IMD to regulate an AF event and the amount of electrical energy required from the IMD to regulate an AF event is dependent upon how close one of the IMD's lead electrodes is positioned relative to the AF driver.
Accordingly, during the IMD implantation process, every effort must be made by the medical practitioner to co-locate the lead electrode with the AF driver. Similar to the previously mentioned ablation procedure, the medical practitioner will attempt to co-locate the lead electrode with the AF driver by comparing ECG signals and intracardiac signals sensed by the lead electrode, as the lead electrode is moved along the interior surface of the heart. It can be difficult for a medical practitioner to co-locate the IMD's lead electrode with the AF driver because the comparison of the electrical signal waveforms can be time-consuming and tedious for the medical practitioner.
It should, therefore, be appreciated that there is a need for a system and a related method for assisting a medical practitioner to quickly and accurately determine the location within a patient's heart where ablation energy should be delivered, or where an IMD lead electrode should be located, during therapeutic procedures associated with the elimination of fibrillation. The present invention satisfies these needs, as well as others as discussed below.